// Given a list postal codes, find the FSA (Forward Sortation Code) code that occurs most frequently.
// input:
// ["M5V 1W5", "L7Q 1T3", "L7M 1W1", "M5V 6X1", "M5V 2X1", "L7M 1W6"]
// Output:
// "M5V"
Second Revision: They told me to support the US portal code.
// Given a list postal codes, find the FSA (Forward Sortation Code) code that occurs most frequently.
// input:
// ["M5V 1W5", "L7Q 1T3", "L7M 1W1", "M5V 6X1", "M5V 2X1", "L7M 1W6", "42015", "11005", "41450", "11026"]
// Output:
// "M5V"
Third Revision: They told me to return multiple values. If multiple values have the same occurrence.
// Given a list postal codes, find the FSA (Forward Sortation Code) code that occurs most frequently.
// input:
// ["M5V 1W5", "L7Q 1T3", "L7M 1W1", "M5V 6X1", "M5V 2X1", "L7M 1W6", "42015", "11005", "41450", "11026", "42016", "42017"]
// Output:
// ["M5V", "420"]